1. Field of the Invention
The present invention relates to self-ignition 4-stroke internal-combustion engines.
2. Description of the Prior Art
Self-ignition is a well-known phenomenon in 2-stroke engines. This combustion type has advantages as regards emissions: low hydrocarbon and nitrogen oxides emissions are notably obtained.
Very low nitrogen oxides emissions can be obtained for 4-stroke engines. Furthermore, a remarkable cycle regularity is achieved during self-ignition combustion.
Self-ignition is a phenomenon which allows initiation of combustion by means of residual burned gases which remain in the combustion chamber after combustion.
Self-ignition is achieved by controlling the amount of residual gases and the mixing thereof with the fresh gases (not burned yet). The residual gases (hot burned gases) initiate the combustion of the fresh gases thanks to a combination of temperature and of the presence of active species (radicals).
In 2-stroke engines, the presence of residual gases is "inherent" in the combustion. In fact, when the load of the engine decreases, the amount of fresh gases decreases and is naturally replaced by an amount of residual gases (burned gases from the previous cycle or cycles which have just flowed out of the cylinder). The 2-stroke engine thus runs with an internal recirculation (or internal EGR) of the burned gases at partial load. However, the presence of this internal EGR is not sufficient to obtain the desired self-ignition running. Research work also shows that the mixing between this internal EGR and the fresh gases is to be controlled and limited.
The controlled self-ignition technology applied to 4-stroke engines is particularly interesting because it allows running of the engine with an extremely diluted mixture, with very low fuel/air ratios and ultra-low NOx emissions.
However, this technology experiences a significant technological difficulty insofar as, in order to obtain self ignition in 4 stroke engines without the internal EGR effect of the 2-stroke engine, it is necessary to either increase the compression ratio of the engine very highly (with knocking problems at high load), or to heat up the admitted fresh gases very highly (several hundred degrees C), or to combine both phenomena.
Solutions which decrease pressure and temperature level requirements for 4-stroke engines can be partly provided by suitable additives in the fuel. French patent application EN. 95/10,798 illustrates such a solution.
It is well-known for 4-stroke engines, for example as disclosed by PCT international application WO. 93/16,276, to combine a variable distribution adjustment with a one-way system at the intake in order to decrease pumping losses at partial load. This solution thus allows the engine to run with the intake throttle valve as wide open as possible.
In a different way, the present invention mainly overcomes the aforementioned problems linked with self-ignition 4-stroke engines.